Investigators have developed a variety of technical approaches to the generation of coordinate pair signals from electrographic devices. Industrial requirements for these devices are increasing concomitantly with the evolution of computer graphics, computer-aided design, and computer-aided manufacturing systems. Thus, a considerable degree of accuracy in pinpointing physical conditions upon the surfaces of the electrographic devices or digitizers is required for many applications.
The operation of a digitizer or graphics tablet generally involves the same manual procedures as are employed in conventional graphics design, generally, a locator or cursor or stylus being drawn across or selectively positioned upon the surface of the graphics tablet. In turn, the tablet responds to the position of the cursor to generate analog paired coordinate signals which are digitized and conveyed to a host computer.
A variety of design approaches have been employed for the fabrication of digitizer structures. The more recent and improved approaches to such designs have been to resort to a capacitive coupling of the cursor or locator instrument with the position responsive surface to generate analog paired coordinate signals. Such capacitive coupling can be carried out either with a grid layer which is formed of spaced linear arrays of conductors or through resort to the use of an electrically resistive material layer or coating.
An immediately apparent advantage of developing position responsive surfaces or digitizers having writing surfaces formed of a continuous resistive material resides in the inherent simplicity of merely providing a resistive surface upon a supportive substrate such as glass or plastic. Further, unlike conventionally encountered grid structures,the resistive coatings as well as their supportive substrates may be transparent to considerably broaden the industrial applications for the devices. For example, the digitizers may be placed over graphics or photographic materials for the purpose of tracing various profiles.
A variety of technical problems have been encountered in the development of resistive coating type digitizer devices, one of which concerns the non-uniform nature of the coordinate readouts achieved with the surfaces. Generally, precise one-to-one correspondence or linearity is required between the actual cursor position and the resultant coordinate signals. Because the resistive coatings as well as the glass substrates upon which they are mounted cannot be practically developed without local resistance (thickness) variations, the non-linear aspect of the otherwise promising approach has required a considerable amount of investigation and development. Exemplary of such development is the boarder treatment or switching technique of Turner in U.S. Pat. No. 3,699,439, entitled "Electrical Probe-Position Responsive Apparatus and Method", issued Oct. 17, 1972, and assigned in common herewith. This approach uses a direct current form of input to the resistive surface from a hand-held stylus, the tip of which is physically applied to the resistive surface. Schlosser et al., describes still another improvement wherein an a.c. input signal is utilized in conjunction with the devices and signal treatment of the resulting coordinate pair output signals is considerably improved. See U.S. Pat. No. 4,456,787 entitled "Electrographic System and Method", issued June 26, 1984, also assigned in common herewith. Position responsive performance of the resistive layer devices further has been improved by a voltage waveform zero crossing approach and an arrangement wherein a.c. signals are applied to the resistive layer itself to be detected by stylus or tracer as described in U.S. Pat. No. 4,055,726 by Turner et al. entitled "Electrical Position Resolving by Zero Crossing Relay", issued Oct. 25, 1977, and also assigned in common herewith. Substantially improved accuracies for the resistive surface-type digitizer devices having been achieved through a correction procedure wherein memory retained correction data are employed with the digitizer such that any given pair of coordinate signals are corrected in accordance with data collected with respect to a grid located position on the digitizer resistive surface during the manufacture of the ditizers themselves. With such an arrangement, the speed of correction is made practical and the accuracy of the device is significantly improved. The correction table improvements for these surfaces is described, for example, in application for U.S. patent, Ser. No. 06/742,733, entitled "Electrographic System and Method", filed June 7, 1985, by Nakamura et al. and assigned in common herewith, now U.S. Pat. No. 4,650,926. An improved signal treatment technique for developing more accurate signal response or linearities from resistive surface type digitizers is described in U.S. Pat. No. 4,600,807, by Kable entitled "Electrographic Apparatus" issued July 15, 1986. The latter patent also describes an improved cursor structure having a canted transparent support for a conductive annular ring which is positioned adjacent the electrographic surface to receive signals emanating therefrom for transmission to the noted front-end treatment electronics. The canted design has been found to be very convenient for the user from an ergometric standpoint.
Capacitive coupling using a stylus or cursor has been employed with grid layers which are formed as adjacent but spaced apart arrays of elongate thin conductors. For example, these conductors may be provided as lines of silver ink deposited in orthogonally disposed relationship upon opposite faces of a sheet of insulative material such as Mylar. A resistance network is employed with each of the conductor arrays such that a predetermined resistance is coupled between each conductor from first to last. A technique considerably improving this form of grid array digitizer wherein the grid arrays themselves are excited by an a.c. signal for data collection from a stylus or cursor is described in an application for United States patent by Kable entitled "Position Responsive Apparatus, System and Method having Electrographic Application", Ser. No. 791,324, filed Oct. 25, 1985, and assigned in common herewith.
The stylus and/or cursor structures which are utilized with digitizer tablets must be designed to avoid error associated with their capacitive coupling to the active surface. This is particularly true in the preferred case of digitizer structures wherein the surfaces thereof themselves are excited by an a.c. current. Generally, the hand and body of the user of the cursor or stylus carries part of this a.c. signal which may generate error induced by the passage of signals from the hand and body of the user into the pick-up itself. Further, early cursor structures have been found to exhibit error in position data acquisition due to the irregularities of the digitizer surface over which they are positioned. For example, should the annular ring employed with a cursor be positioned closer to the tablet surface at one diametric extent as opposed to the opposite, then a variation in capacitive coupling is developed. This variation will tend to offset the electrical center of the cursor with respect to the mechanical center which is developed by crosshairs extending through the center of the annulus and visible by the operator. Error also has been occasioned with the use of conventional cursors having an annulus or ring pick-up where the devices are manipulated near to the periphery or borders of the digitizers tablets. Because these borders generally are at ground, the pick-up characteristic of the cursors commences to assume a step-type function output as they are influenced by the change in signal represented by the border condition. Very often, error to the extent of 0.300 inch will be experienced from these border step function effects or through interaction of the cursor device with the body of the user itself.